  |
|
|
|
Species belonging to the phylum Microspora are called microsporidia. These organisms appear to have separated very early from the eukaryotic family tree. They have true nuclei, but no mitochondria or peroxisomes. Their ribosomes are prokaryote-like. However, they can produce some typical mitochondrial-like proteins such as mitochondrial heat shock protein. In the cytoplasm one can recognise some mitochondrial remnants, suggesting that the parasite lost mitochondria during evolution and lost them later, rather than never acquiring them in the first place. Some researchers regard them as aberrant fungi. They are obligate intracellular parasites, and are recovered in countless widely varying host groups (insects, fish, rodents, and so on). They have long been know to be pathogenic and are the cause of significant damage in, for example, silk worms (pébrine [
see Pasteur and the ravages of the French sericulture in the 19th Century]), apiculture and fish farms. Species which can parasitise humans are very small (1-2 m m). Encephalitozoon cuniculi holds the record at present for the smallest eukaryotic genome (<2.9 Mb). Other species known to infect humans are Brachiola vesicularum, Encephalitozoon cuniculi, Encephalitozoon hellem, Encephalitozoon intestinalis (previously Septata intestinalis), Enterocytozoon bieneusi, Microsporidium africanum, Microsporidium ceylonensis, Nosema algerae, Nosema connori, Nosema ocularum, Pleistophora sp, Trachipleistophora hominis, Trachipleistophora anthropophthera and Vittaforma corneae (previously Nosema corneum). These organisms have mainly been described in immunodeficient persons.
The parasites have a very characteristic ultrastructure. The organism forms oval-shaped spores with an external exospore (glycoproteins) and an internal endospore (chitin). Within the spore is a coiled spiral tube (polar tube). After it is ingested, the spore is stimulated to protrude this polar tube which then penetrates a host cell. The sporoplasm is then injected via this tube into the cytoplasm of the host cell. Subsequently there is reproduction of the parasite (merogony and sporogony). New spores may infect other neighbouring cells or be passed to the outside world to infect a new host. An alternative way of infecting host cells is via endocytosis of the spore by the host cell. After fusion with the lysosome, the spore can eject her filament and escape from the deadly vesicle.
Transmission is chiefly via the faeco-oral route, but much is still uncertain. Transplacental transmission occurs in carnivores, but has not yet been described in humans. Possibly transmission is via aerosol for those protozoa which cause corneal lesions. Transmission via infected water is being investigated.
Symptoms will be determined by the anatomical location of the parasites. Disseminated infections, corneal infections (keratitis), intestinal locations etc. all occur. In HIV patients with low resistance (CD4 < 100/m L), there is often persistent diarrhoea, abdominal pain, loss of weight and sometimes sclerosing cholangiopathy.
Diagnosis by light microscope (faeces, biopsies, corneal scraping) is often difficult due to the small dimensions of the parasites and the labour-intensive staining techniques. New staining techniques, such as the fluorochromes calcofluor, Fungifluor or Uvitex 2B and modified trichrome staining (modification according to Weber or Kokoshin), are now available. Experience is essential and the parasites must be properly differentiated from fungal spores and bacteria. Electron microscopy is a good technique for species identification, together with PCR [polymerase chain reaction], but of course this can only be carried out in specialised centres.
There is still too little known about treatment. Fumagillin is a product originating from Aspergillus fumigatus which is used in microsporidiosis of honey bees. It has been used topically in keratitis with good results. Other drugs have been used with varying success. Albendazole is effective in infections with Encephalitozoon intestinalis and to a lesser extent in Enterocytozoon bieneusi. Nitazoxamide possibly has a place in treatment. Improving immunity in HIV patients, e.g. by combination antiretroviral therapy, may lead to remission of the infection. For symptomatic treatment (e.g. in persistent diarrhoea without knowing its cause), loperamide (Imodium®), opioids (laudanum) or even somatostatin analogues may be used. The latter is of course not easily available in developing countries.
